The current developmental trend for wireless networks is increasing the data rate. As the data rates increase, the number and types of applications that can use the wireless networks also increases. For example, in the early wireless networks, applications were typically limited to file transfers and other computer data related operations because of bandwidth limitations and latency issues. Time critical applications, such as streaming audio and video, usually require more bandwidth than files and computer data (along with less latency and jitter) and normally could not be serviced by these early wireless networks. Still, the convenience of being wireless made these early wireless networks a viable product despite the limited functionality. Now, due to the introduction of new wireless networks with significant data rates, wireless networks usage models are being expanded to include applications such as multimedia distribution (video and/or audio), video conferencing, and so forth, which require much higher data rates than previous applications.
Since applications such as multimedia distribution, video conferencing, and so forth, are interactive in nature, the performance of the network used in carrying the information is crucial. The key issues for audio and video streaming over a wireless network are latency and jitter. Latency and jitter can affect the timing of when the audio and/or video frames are received at a decoder and presented to the user. Due to the nature of video and audio codecs, a receiving decoder must receive the audio and/or video stream with the same packet timing relationships as was present at an originating source. If the original packet timing relationships of streams from the source (also referred to as a source device) are not preserved at the receiver (also referred to as a sink device), then the receiver's decoder buffer may overflow or underrun. This can cause audible and visible glitches and provide a poor user experience. These problems were not significant issues when wireless networks were used solely for data, which can be far less delay and jitter sensitive, but are issues that must be addressed for streaming audio and video applications.
A technique that can be used to help ensure that frames arrive in a timely manner is to impose quality of service (QoS) constraints on at least a portion of the communications system. By implementing QoS, frames that are timing crucial can be given priority over those that are not. This can ensure that the higher priority frames will arrive at their destination with minimum latency.
A technique that can be used to preserve packet timing relationships between the source device (transmitter) and the sink device (receiver), and thereby minimize the effects of latency and jitter over the wireless network, is to place time stamps on the transmitted frames. When a frame is received at the receiver, the receiver can retrieve the time stamp from the frame and release the frame to the application once the local clock reading reaches the value in the time stamp.
One disadvantage of the prior art is that while the implementation of QoS constraints can help to ensure timely delivery of transmitted frames, the QoS constraints typically do not contain mechanisms for ensuring that the inter-arrival patterns between packets are retained across the wireless network.
A second disadvantage of the prior art is that while time stamping is used in controlling the release of received frames, there is usually a clock rate mismatch between the transmitter and the receiver. Hence, problems such as buffer overflow and underrun can still occur since the clocks of the transmitter and receiver can continue to drift further and further apart.